Thermal radiation light sources have the advantage that an emission can be obtained by merely imparting heat to an object. For example, in a gas sensor for analyzing the components in an exhaust gas from an engine, the thermal radiation light source can be preferably used as a light source for converting waste heat produced by the engine into infrared rays for the sensing.
Electromagnetic waves emitted from an object which has been given heat have a spectrum spreading over a wavelength range dependent on the temperature. For example, electromagnetic waves generated by heating an object to a temperature of tens to hundreds of degrees Celsius have a wavelength range of a few μm to several tens of μm, and as the temperature rises, the range spreads to the shorter wavelength side. However, since the aforementioned infrared sensor normally utilizes only infrared rays of a specific one or more wavelengths, using such a thermal radiation light source causes the measurement target to be irradiated with infrared rays of unnecessary wavelengths other than the specific wavelength, which produces adverse effects, such as the heating of the measurement target. In the case of generating the thermal emission by supplying electric energy, using an emission source which generates a broadband emission causes the problem of an increase in the power consumption.
In order to solve such a problem, Patent Literature 1 has offered a thermal radiation light source including a quantum well structure in a photonic crystal. The photonic crystal includes a periodic refractive index distribution and is capable of forming a standing wave of light having a specific wavelength corresponding to the period. In the Patent Literature 1, the photonic crystal includes different refractive index regions (typically, holes) having a refractive index different from a refractive index of the substrate wherein the different refractive index regions are arranged on a plate member. By arranging the columnar members (different refractive index members) periodically on the base (in the air), it is possible to use parts on the base (air and different refractive index members) as a photonic crystal. A quantum well structure is the structure of an object in which an energy potential in the form of a well (“quantum well”) is created by stacking a plurality of kinds of semiconductor layers with a thickness of approximately a few nm to a dozen nm, with each layer having a different magnitude of energy band gap.
In this thermal radiation light source, when heat is supplied from a heat source, a transition (intersubband transition) occurs between a plurality of discrete energy levels (subbands) formed in a quantum well having a quantum well structure. Light emission having a finite bandwidth centered on the wavelength corresponding to the transition energy occurs. In the photonic crystal provided with the quantum well structure, light having one wavelength determined by the period of the photonic crystal resonates and is amplified, and light having a wavelength spectrum having a sharp peak at the one wavelength is generated.
The thermal radiation light source further includes an electrode for applying a voltage to the quantum well structure. By turning ON/OFF the application of this voltage, the number of electrons or positive holes in the quantum well can be changed, whereby the intensity of the light having the specific wavelength can be controlled.